1. Field of the Invention
The present invention relates to an apparatus and method for manufacturing a vitreous silica crucible, which is capable of improving a yield of a crucible by reducing contamination generated during fabrication of a vitreous silica crucible, while enabling stable operation for a long time by protecting a main body of an apparatus.
2. Description of Related Art
Currently, growing of silicon single crystal constituting a raw material of a semiconductor wafer is mainly performed by using a Czochralski (CZ) method. In the Czochralski method, a seed crystal is dipped in a silicon melt accommodated in a crucible, and the seed crystal is pulled up while being rotated, thereby growing a single crystal from below the seed crystal.
In the growing of the silicon single crystal, a vitreous silica crucible is generally used so as to avoid impurities being mixed with the silicon melt. Specifically, in order to grow a high purity semiconductor crystal, or to precisely control concentration of a dopant added to a semiconductor, it is required to precisely control purity of an inner surface of a vitreous silica crucible, which contacts the silicon melt. Also, in order to improve a yield of silicon single crystal growth by continuously pulling up a plurality of single crystals, the vitreous silica crucible needs to have durability to endure long use or repetitive use.
An arc fusing method is used as a method of manufacturing a vitreous silica crucible. In this method, a deposit layer of silica powder is formed in a predetermined thickness on an inner surface of a mold having a bottom and a cylindrical shape, and powder molded body approximately corresponding to a shape of a target crucible is formed. Then, the deposit layer of the silica powder is vitrified by heating and fusing the deposit layer via a discharge through an arc electrode installed on the mold, while rotating the mold. According to the arc fusing method, since the silica powder can be fused without being contacted while limiting a heated region, contamination due to impurities in an inner surface of a vitreous silica crucible may be suppressed, thereby providing a high quality vitreous silica crucible.
However, even by using the arc fusing method, generation of silica fume generated as part of silica powder is evaporated during fusing of the silica powder is unavoidable. Specifically, a phenomenon called dropping, wherein the silica fume that adheres to and coheres to a surface of an electrode, drops onto a molten glass surface, and adheres to a crucible inner surface, has become a problem.
Also, a carbon rod is generally used as the arc electrode, but carbon particles detached when an electrode surface combusts by arc discharge tend to drop onto the molten glass surface. When the carbon particles that are dropped onto the glass surface are combusted in that spot, the carbon particles cause unevenness, and when the carbon particles remain in that spot as carbon particles, the carbon particles become impurities, thereby deteriorating quality of the vitreous silica crucible.
As countermeasures, a technology for controlling characteristics of a material of the carbon rod constituting the arc electrode so as to suppress cohesion of the silica fume or dropping of the carbon particles is being studied.
For example, JP-A-2007-273206 discloses a technology for controlling density of a carbon electrode to be within a predetermined range (1.60 g/cm3 to 1.80 g/cm3) and controlling a particle size of carbon to be less than or equal to 0.05 mm to suppress adhesion of silica fume to an electrode surface, thereby preventing dropping of the carbon particles onto the molten glass surface.
A technology of preventing leakage generally by setting a generation source to negative pressure under an environment where fume is generated is well known (for example, JP-A-2002-241077). However, in a process of manufacturing a vitreous silica crucible, external dust intruding into a mold also causes contamination, and thus such a technology is not preferable. To avoid this intrusion of dust, for example, JP-A-2003-313035 discloses a technology for sealing a section where a mold is installed, and circulating clean air in the section by supplying and exhausting air.